Stroke. Coloured Positron Emission Tomography (PET) scan of the brain of a stroke patient. This is a horizontal section through the brain. Colour- coding is: high brain activity (red to yellow); low activity (green to blue). At centre left is a lesion (green/light blue) showing an area of brain damage with reduced blood flow and low activity due to stroke. Stroke, or cerebrovascular accident (CVA), is a rupture or blockage of a blood vessel causing brain damage such as loss of speech or paralysis usually affecting one side of the brain. In PET scanning a radioactive tracer is injected into the blood to reveal activity in the brain.